In the evolution to higher power levels in audio amplifiers, matters of weight, size, reliability, cost, etc., and their tradeoffs have had to be re-evaluated. Class H operation of the power stages has been utilized for increased efficiency, requiring a dual power supply delivering a main voltage supply (e.g. .+-.43 volts) and a higher auxiliary voltage supply (e.g. .+-.100 volts) that is called upon for additional headroom on peak power demands. Class H operation has been successfully utilized in high power audio amplifiers powered in the conventional manner from a basic 60 Hz transformer-type power supply. The main disadvantage of this approach is excessive weight and size, due mainly to the 60 Hz power transformer.
Weight and size reduction are available though the use of a "switch-mode" power supply, i.e. an AC/DC/ac/dc converter where the first AC to DC conversion from the 120 volt power line is transformerless, utilizing a full wave rectifier bridge to provide DC at about 170 volts. This powers a generator that supplies an HF (high frequency) ac output through a relatively small, lightweight HF transformer and bridge rectifier(s) supplying the output dc; this HF transformer, which can be made very light and small, serves to electrically insulate and thus isolate the ac input power line from the common ground of the power supply load (and of the amplifier) in order to prevent ground currents and eliminate shock hazard.